1. Field of the Invention
This invention relates to a toner for developing electric latent images or magnetic latent images in electrophotographic processes, electrostatic printing processes and the like, and more particularly, to a capsule toner suitable for pressure fixation.
2. Description of the Prior Art
Heretofore there have been known various electrophotographic processes such as those disclosed in U.S. Pat. No. 2,297,691, British Pat. No. 1,165,406, and British Pat. No. 1,165,405. These processes usually comprise utilizing a photoconductive material, forming electric latent images on a photosensitive member by some means, developing the latent images with a toner, if desired, transferring the developed toner images to a receiving sheet such as paper and then fixing the toner images by heat, pressure, or solvent vapor.
There are known various methods for visualizing electric latent images with a toner. For example, there may be mentioned magnetic brush development as described in U.S. Pat. No. 2,874,063, cascade development as described in U.S. Pat. No. 2,618,552, powder cloud development as described in U.S. Pat. No. 2,221,776, fur-brush development, liquid development and the like. Toners used for these development methods are heretofore fine powders composed of dyes or pigments dispersed in natural or synthetic resins. If desired, third components are added to the fine powders.
Developed toner images may be transferred to a receiving sheet and fixed, if desired.
As a method of fixing toner images, there may be mentioned a method comprising heating and melting toner particles by a heater or heat roller and fusing followed by solidifying on the support, a method for fixing toner particles to a supporting member by softening or melting the resin binder in toner particles with an organic solvent, a method for fixing toner particles to a supporting member by pressure, and the like.
Materials for toner particles are selected in such a way that they are suitable for each particular fixing method. Therefore, a toner suitable for a particular fixing method is usually not usable for the other fixing methods.
In particular, a toner used for widely used, conventional heat fusing fixing methods employing a heater is hardly possible to be applied to a heat roller fixing method, a solvent fixing method, a pressure fixing method and the like. Therefore, toners suitable for each particular fixing method are researched and developed.
The method for fixing toners by pressure is for example, disclosed in U.S. Pat. No. 3,269,626 and Japanese Patent Publication No. 15876/1971. The method has various advantages, that is, less consumption of energy, non-pollution, copying without a waiting time by simply switching on a copying machine, no fear of burning and scorching copies, high speed fixing and a simple fixing device and the like.
However, the pressure fixing method has some disadvantage such as poor fixability of a toner, offsetting to the pressure roller and the like. Therefore, researchers have been made to improve the pressure fixation. For example, British Pat. No. 1,210,665 discloses a pressure fixation toner containing an aliphatic component and a thermoplastic resin; U.S. Pat. No. 3,788,994, U.S. Pat. No. 3,974,078, Japanese Patent Laid Open Nos. 17739/1974 and 108134/1977 disclose pressure fixable toners of a capsule type containing a soft material in the core; and Japanese Patent Laid Open No. 75033/1978 discloses a pressure fixable toner composed of a block copolymer derived from a sticky and strong polymer and a soft polymer.
However, any practical pressure fixable toner is not yet obtained which can be easily produced, has a sufficient pressure fixability, does not cause offsetting to the pressure roller, is stable in points of developing property and fixability upon repeating use, does not adhere to carrier, metal sleeve and surface of the photosensitive member and has a good storage stability, i.e. non-agglomerative and non-caking.
For example, a pressure fixable toner comprising a soft material is of good pressure fixability, but is disadvantageous because this type of toner can not be easily pulverized to toner particles, easily causes offsetting to a pressure roller and adhering to a carrier and a surface of photosensitive member and causes agglomeration and caking during storage.
Rigid resins can be used to make easily toners and the resulting toners are of good chargeability and storage, but have a very poor pressure fixability because most rigid resins are harder than cellulose fibers constituting papers and when they are pressed, they are not entangled with the fibers, but simply crashed in the paper.
When a soft material of good pressure fixability is used as a core material in conventional pressure fixable capsule toners, the soft material gradually adheres to the pressure roller during repeating pressure fixation and, therefore, this disadvantageously causes offsetting and the receiving paper is adhesively wound round the roller.
If one tries to avoid such phenominon, the pressure fixability is lowered. In short, a toner of high pressure fixability gives poor image quality while a toner capable of giving good image quality is poor at fixation.
Conventional capsule toners are easily broken even by a slight impact and the outer shell and the core are separated from each other, and the like is short and the resulting image quality is poor.
There has been recently used a method of developing electrostatic images with a one-component developer which has toner particles containing magnetic fine powder and does not use any carrier particles. In this method the toner binder resin is required to have good dispersibility and contacting property with respect to the magnetic fine powders and the toner particles are required to have high impact strength and fluidity. In addition, when the core material and the shell material are separated from each other during development carried out by triboelectric charge caused by the one-component developer and the developing sleeve roller, the shell material is adhered to the sleeve roller by triboelectric force and accumulated, and therefore the durability is very poor. In view of the foregoing, practical capsule toners have not yet been provided.